With respect to the efforts which are being made to provide airplanes which conform to future ecological requirements and are inexpensive to produce and operate, and to nevertheless meet the strictest safety requirements, possible ways are increasingly being sought to produce the essential primary structures (e.g. wings, fuselage components, housing for the drive units, etc.) using fiber-reinforced composite material rather than aluminum. This lightweight construction technique makes it possible, in particular, to considerably reduce the weight of the airplanes. During the production of such essential primary structures, it must be taken into account that these take on a considerable scale; by way of example, the landing flaps are structural parts which extend over a number of meters. These structural parts are additionally exposed to high levels of stress during operation and therefore represent safety-critical structural parts, for which special quality requirements have to be observed.
Fiber-reinforced composite materials of this type generally comprise two essential components, namely firstly the fibers and secondly a polymer matrix which surrounds the fibers. The matrix encompasses the fibers and is cured by a thermal treatment (polymerization), such that three-dimensional cross-linking takes place. This polymerization has the effect that the fibers are bonded firmly to one another and therefore forces can be introduced into the fibers, namely predominantly via shear stresses. Suitable fibers are both carbon fibers and possibly also glass fibers. Carbon fibers, which nowadays are still relatively expensive, regularly consist of carbon to an extent of at least 90% by weight. The diameter of the fibers is, for example, 4.5 to 8 μm (micrometer). Carbon fibers of this type have anisotropic properties. By contrast, glass fibers have an amorphous structure and isotropic properties. They predominantly comprise silicon oxide, it being possible for further oxides to be admixed if appropriate. Whereas the glass fibers are relatively inexpensive, the carbon fibers are noted for their high strength and rigidity.
Particularly in the construction of airplanes, what is known as pre-preg technology is employed. In this technology, for example, pre-impregnated fabrics or other fiber forms (pre-form) are soaked in synthetic resins and thermally treated merely until they solidify slightly (gel formation), such that they can be handled in layers. A pre-preg material of this type exhibits a small degree of adhesion and can therefore be arranged readily in appropriate molding tools or one on top of another in layers, until the desired form of the structural part is formed. When the desired layers of the pre-preg material are arranged, they can be (thermally) cured. In order to cure said pre-preg structural parts, use is presently made of what are known as autoclaves, i.e. ovens which may have to be heated with an overpressure (up to 10 bar) over many hours in order to achieve complete curing of the structural parts.
In addition, DE 10 2005 050 528 A1, the contents of which are incorporated by reference, discloses a microwave autoclave, with which the production of fiber composite structural parts by microwave radiation is proposed. The apparatus proposed in said document makes it possible to couple microwave radiation into the pressure chamber of the autoclave. The excitation of the pre-preg materials with microwaves has the advantage that it is not necessary to heat the air located in the autoclave or the inert gas located therein, which is present in a considerable volume owing to the size of the structural parts. The use of microwave technology makes it possible to heat the material to be cured itself directly, and the rest of the surrounding region accordingly remains relatively cold. When heating the pre-preg material using microwaves, the following active mechanisms may set in depending on the material used: dielectric heating and resistive heating. Long-chain hydrocarbon molecules (such as e.g. in epoxy resin) are dipoles (i.e. have an irregular charge distribution) and are excited to oscillate at a high frequency in the electromagnetic field produced by the microwaves. This kinetic energy of the dipoles is then converted by internal friction into heat, which is produced directly in the material (dielectric heating). In addition, it is also possible for eddy currents to arise as a result of induction, and therefore the electrical resistance of the material finally causes an increase in temperature (resistive heating). By way of example, the material can thus be heated to temperatures above 130° C. or even above 160° C., a temperature at which the polymerization or curing of the pre-preg materials regularly begins.
The microwave resonator described in DE 103 29 411 A1, the contents of which are incorporated by reference, is likewise suitable for carrying out such a thermal treatment. Said microwave resonator is generally operated without an overpressure. However, it may also be integrated, if appropriate, in a pressure vessel (autoclave).
A problem which arises during the curing process for such large structural parts, as are used in the construction of airplanes, is that possibly more complex geometries of the components require additional processes for joining such fiber-reinforced composite materials. For this purpose, it was customary to bond cured structural parts to one another by joining using a bonding agent. For this purpose, the surfaces of the cured structural parts were treated, if appropriate, for example ground and/or cleaned. Then, an adhesion promoter was applied, under certain circumstances, to the treated surfaces. This was followed by the application of an adhesive, with which the structural parts to be bonded to one another were then fixed. It is not just the case that this process necessitates relatively laborious handling of the large structural parts; in addition, the pretreatment of the structural parts and also the joining process itself have to be carried out very precisely because here faults repeatedly lead to weakening of the structural parts, which should not be accepted particularly in the construction of airplanes.